Browsing by Author "V.K. Wadhawan"

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Crystallographic and Microstructural Aspects of the Superconductor YBa2Cu3O7-y —An Overview
(1990) Dhananjai Pandey; V.S. Tiwari; A.K. Singh; V.K. Wadhawan; M.S. Somayazulu
Crystal structures and structural inhomogeneities observed in YBa 2 Cu 3 0 7 _, are reviewed. It is brought out that a proper understanding of the nature of the structural inhomogeneities is essential for exploiting the technological potential of this material. The need for an adequate characterization of specimens used for experiments is emphasized. Practical implications of the ferroelastic nature of the material are discussed. © 1990, Taylor & Francis Group, LLC. All rights reserved.
Kinetics of formation of the pyrochlore and perovskite phases in sol-gel derived lead zirconate titanate powder
(Materials Research Society, 1998) V.S. Tiwari; Arun Kumar; V.K. Wadhawan; Dhananjai Pandey
Lead zirconate titanate (PZT) powder is prepared by the sol-gel method. The formation of pyrochlore and perovskite phases is investigated by high temperature x-ray diffraction (XRD) and thermal analysis techniques. The pyrochlore phase first appears in x-ray amorphous form, and then gets converted to crystalline state on annealing in air. We show that vacuum annealing of the pyrolyzed amorphous PZT gel suppresses the formation of the crystalline pyrochlore phase. This, in turn, enhances the kinetics of conversion of pyrochlore to perovskite, such that a pyrochlore-free perovskite phase can be obtained by annealing at about 500°C. On the other hand, if annealing is carried out in air, a crystalline pyrochlore phase is formed, which requires annealing temperatures higher than 600°C for transformation to the perovskite phase. These observations are explained tentatively in terms of the oxygen stoichiometry of the two phases.